1. Field of the Invention
The invention relates to an antenna device and, more particularly, to an antenna device having a circuit that switches the characteristic of an antenna.
2. Description of the Related Art
In a related art, there is an antenna device that adjusts the resonant frequency by changing a voltage applied to a varicap diode to thereby switch the frequency of radio waves that are transmitted or received. In an antenna device that includes an antenna element and a resonant circuit formed of a varicap diode and an inductor, where the capacitance of the varicap diode is “C”, and the inductance of the inductor is “L”, the resonant frequency fRES is expressed as follows.fRES=1/{2π(L·C)1/2}Here, the capacitance C of the varicap diode varies with a voltage applied to the varicap diode. As the voltage increases, the capacitance C decreases. Thus, the resonant frequency of the antenna device may be changed by utilizing the characteristic such that, by increasing the voltage, the capacitance C decreases and, therefore, the resonant frequency fRES increases.
Japanese Patent Application Publication No. 2002-76750 (JP-A-2002-76750), for example, describes an antenna device of which the resonant frequency is variable using the varicap diode. In JP-A-2002-76750, as shown in FIG. 9, the antenna device includes an antenna conductor 35 and an LC parallel resonant circuit 34, which are connected in series with each other. The LC parallel resonant circuit 34 has an inductor 32 and a varicap diode 31. When a voltage is applied to a voltage input terminal 33, the voltage is applied to the varicap diode 31, and then the capacitance of the varicap diode 31 varies. Thus, the resonant frequency is variable.
However, in the resonant antenna of which the resonant frequency is variable using the varicap diode according to the related art, it is necessary to apply the varicap diode with an appropriate voltage corresponding to a resonant frequency. That is, in accordance with the frequency of radio waves that are intended to be transmitted or received, a voltage applied to the varicap diode needs to be controlled to an appropriate value. Thus, an additional device for controlling a voltage applied to the varicap diode is necessary. This increases the size of the antenna device and, as a result, increases manufacturing costs.